Effect of Flow Rate on the Corrosion Behavior of P110 Steel in High-Ca²⁺ and High-Cl⁻ Environment

Abstract

With the exploitation of oil and gas resources, the water environment of high-Ca²⁺ and high-Cl⁻ stratum puts forward high safety requirements for tubular columns. This paper simulates the underground environment by using high-temperature and high-pressure autoclaves, combines electrochemical research results, and analyzes the effect of flow rate on the corrosion behavior of P110 steel in a water environment of 7.5 g/L Ca²⁺ + 128 g/L Cl⁻ simulated stratum. The research results show that the presence of Ca²⁺ promotes the acidification of the solution and accelerates the dissolution of P110 steel. With the increased flow rate of the fluid, the corrosion rate of P110 steel increases, but the increasing trend slows down gradually. At the same time, the flow rate decreases the probability of corrosive pitting on P110 steel. The decreasing is closely related the peeling of earlier CaCO₃ precipitation by the fluid.

1. Introduction

As the exploitation of oil and gas enters the high water-cut stage, the contents of Ca²⁺, Cl⁻, and bacteria being explored from water increase. For example, the mineralization of Zhongyuan and Qinghai Oil Fields has reached 100,000 mg/L [1], which poses a threat to the safe operation of the injection–production system. At present, studies on the corrosion behavior of pipe columns and pipe networks in solutions containing Ca²⁺ ions have not drawn consistent conclusions. Some investigators have revealed that the addition of Ca²⁺ ions increased the corrosion rate but resulted in a loss in the structure of the product scale after Ca²⁺ was substituted by Fe²⁺, while others hold the view that the addition of Ca²⁺ ions decreased the corrosion rate of the specimens due to the precipitation of Ca_xFe_1−xCO₃, which fills the pores in the product and strengthens the passivation-like characteristics of the product scale [2,3,4,5,6,7,8,9,10,11,12,13]. Some reports have also revealed that adding Ca²⁺ ion triggers the pitting of materials, but others have shown that adding Ca²⁺ ions delays pitting corrosion [14,15,16,17]. The main reason for these differences is that the effect of Ca²⁺ on the film growth mechanism when the water chemistry is changed by the addition of Ca²⁺ ions has not been clarified.

A high flow rate increases the transmission rate of the medium, destroys the surface scale of the specimen, and increases the corrosion rate of the specimen [18,19,20,21,22,23,24,25,26,27,28,29,30,31,32]. However, a high flow rate may also promote the formation of a hardened surface layer on the samples and slow down the corrosion damage [33,34]. The corrosion rate of metal first increases and then decreases with as the flow rate increases. [35,36,37,38,39,40] A statistical investigation of the effects of flow rate on the corrosion behavior of metal is shown in

Table 1. The investigation of flow rate on the corrosion behavior of metal.

Material	Temperature /Pressure	Concentration of Ca2+/Mg2+/Cl− in Test Solution	Velocity	Reference
T95, Q125	38 and 71 °C, 41.37 MPa CO2 and methane	2% NaCl	0–600 rpm	[14]
stainless steel	Room temperature, 150 °C	Feed water	0.39–4.73 m/s	[19]
X70	80 °C, 10 MPa CO2	Deionized water	0–2 m/s	[20]
X65	-	Oil/water	Static condition	[21]
316 L	565 °C	Nitrate-nitrite salts	0.6–2.0 m/s	[22]
13 Cr, N80	60 °C, 0–5 MPa CO2	44 mg/L Ca2+ + Mg2+, 15333 g/L Cl−	7.47, 10.84 m/s	[23]
AZ91D	-	165 mg/L Cl−	1.77–5.31 m/s	[24]
Q235B	20–40 °C,	0.5 wt.% quartz sands + 3.5%NaCl	6–10 m/s	[25]
P110	60 °C, 5 MPa CO2	1.2 g/L CaCl2, 2.08 g/L NaCl	0–10 m/s	[26]
N80	100 °C, 0.6 MPa CO2	0.88 g/L Ca2+, 32.92 g/L Cl−	0–3 m/s	[27]
Carbon steel	40–60 °C	Crude palm oil	1–3 m/s	[29]
N80	-	3.5%NaCl	0–600 rpm	[31]
Mild steel	50 °C	200 mg/L CaCO3, Chlorides 20 mg/l	0.15–0.45 m/s	[33]
Mild steel	50 °C	3.1 g/L Ca2+ + 1.1 g/L Mg2++ 25 g/L Cl−)	0.19–0.45 m/s	[34]
P110	-	3%NaCl solution	3–6 m/s	[35]
20# steel	100 °C, 1 MPa CO2	3% NaCl	0–6 m/s	[38]
P110	90 °C, 25 MPa CO2	6.0 g/L Ca2+, 110 g/L Cl−	0–2.0 m/s	[39]
13CrSS	180 °C, 27.5 MPa CO2+ H2S	7 mg/L Ca2+, 4646 mg/L Cl−	0, 2.5 m/s	[46]
TiN	-	3.5% NaCl	0.5–2.5 m/s	[47]
X65 Steel	-	3.5% NaCl	Static/flow condition	[48]

. The damage caused by the flow rate to the surface of the materials predominantly depends on the texture and surface state of the materials [18,41,42,43] as well as the flow state [19,44,45], flow rate [18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40], temperature [40,43], and other related factors [46,47,48,49]. However, there are few studies on the corrosion behavior caused by flow rate on tubular column materials in high-mineralization water [32].

In this paper, high-temperature and high-pressure autoclaves are used to simulate the underground environment, and the common oil-well tubular steel P110 is investigated to analyze the effect of the rotation rate on the corrosion behavior. The corrosion products on the surface of the specimen are analyzed using SEM and XRD. The influence mechanism of Ca²⁺ on the rotation rate of P110 steel is analyzed using an electrochemical workstation.

2. Materials and Methods

2.1. Material

The material used in the experiment is the common tubular material P110 steel. The material composition is shown in

Table 2. Chemical composition of P110 steel.

Element	C	Si	Mn	P	S	Cr	Mo	Ni
Content	0.28	0.26	0.60	0.012	0.0025	1.00	0.16	0.016
Element	Nb	V	Ti	Cu	B	Al	N	-
Content	<0.001	0.0046	0.0024	0.052	0.0003	0.0021	-	-

, and the microstructure is shown in Figure 1. The P110 steel is composed of sorbite and upper bainite. Some carbides can be observed.

2.2. High-Temperature and High-Pressure Experiment

The size of the specimen was 50 mm × 10 mm × 3 mm. All specimens were abraded with 400, 600, 800, and 1200 grit silicon carbide papers, degreased with acetone, washed with anhydrous ethanol, and dried with high-purity nitrogen gas (99.999% purity, v/v). Test pieces with defects after grinding were not used.

C276-type high-temperature and high-pressure autoclaves were used in the high-temperature and high-pressure corrosion experiment. A schematic diagram of the equipment is shown in Figure 2. The morphology of the corrosion scales was analyzed using the JEOL-6390A scanning electron microscope. The phase composition of the products was analyzed using the Shimadzu XRD-6000. The experimental parameters are shown in

Table 3. Experimental parameters of the high-temperature and high-pressure corrosion of P110 steel.

Item	Parameter
Experiment time (h)	168
Experiment temperature (°C)	120
Total pressure (MPa)	10
Atmosphere	CO2
Experimental solution (g/L)	7.5 g/L Ca2+ + 128 g/L Cl−
Flow rate (r/min)	0, 60, 120, 180

.

In the high-temperature and high-pressure experiment, the specimens are fixed on a holder, and 800 mL of simulated solution is injected into the autoclaves. The liquid level of the corrosion solution was 2–3 cm higher than that of the top of specimens to ensure that the surfaces of all of the specimens were submerged in solution during the whole experiment. High-purity N₂ was injected into the autoclaves via the gas inlet at a 30 mL/min velocity for 12 h to ensure successful deoxidation. CO₂ and N₂ were added to the predetermined pressure (CO₂: 1 MPa, total pressure: 10 MPa) at 120 °C. Then, the experiment was started.

All of the specimens were removed from the autoclaves after corrosion testing. A cleaning solution (10% vol. HCl + 90% vol. H₂O + 5 g/L C₆H₁₂N₄) was used to remove the corrosion product. The cleaning solution was stirred vigorously until corrosion products were completely removed. The cleaned specimen was washed with tap water immediately and neutralized by immersion in saturated NaHCO₃ solution for 3–5 min. Then, the specimen was rinsed with distilled water, cleaned in anhydrous ethanol, and dried under a N₂ gas stream.

The uniform corrosion rate (mm per year) was calculated from the specific weight loss using the following equation:

$$V_{corr}=\frac{87,600\times \Delta m}{At\rho }$$

(1)

where V_corr, Δm, ρ, A, and t are the specimen corrosion rate (mm y⁻¹), mass loss (g), specimen density (g/cm⁻³), exposed specimen area (cm²), and testing time (h), respectively.

2.3. Electrochemical Experiment

Steel specimens, which were machined into a 10 mm × 10 mm square shape with a thickness of 5 mm, were used for electrochemical measurements. The steel specimens were sealed with epoxy resin, leaving an exposed face of 100 mm². All specimens were ground with 400-, 600-, 800-, and 1200-grit silicon carbide papers, degreased with acetone, washed with anhydrous ethanol, and dried with high-purity nitrogen gas (99.999%).

Electrochemical measurements were performed using a PARSTAT2273 electrochemical system on a three-electrode cell, where the P110 specimen was used as a working electrode (WE), a carbon rod was used as a counter electrode (CE), and a saturated calomel electrode (SCE) was used as a reference electrode (RE). A salt bridge was used to avoid chloride contamination from the RE to the solution. Prior to measurements, high-purity N₂ was injected into solution at an approximate velocity of 50 mL/min for 3 h, and then CO₂ gas was injected for 3 h to ensure a saturation state of the CO₂ in the solution. The potentiodynamic polarization curve was measured at a potential scanning rate of 1 mV/s. The EIS measurements were conducted and, a disturbance signal of 10 mV and a frequency ranging from 1 × 10⁵ Hz to 10⁻² Hz were found. Each test was repeated at least three times to ensure the reproducibility of the results.

3. Results

3.1. Effect of Rotation Rate on the Weight Loss Rate

Considering the error in the weight loss experiment, three pieces of specimen were used in each experiment to calculate the corrosion rate. As showed in Figure 3, the corrosion rate of the P110 steel specimen is lowest in the static environment (0.09 mm/a). With an increasing rotation rate, the increase in weight loss rate slows down and reaches its peak (0.2077 mm/a) at 120 r/min, though the matching degree between the rotating rate of the test device and the real flow rate of the solution is lowered due to the extremely high rotating speeds of the test device, and the weight loss rate at 180 r/min (about 0.75 m/s) is lower than that at 120 r/min (about 0.5 m/s), thereby indicating that the promotion of the rotation rate on the corrosion to P110 steel is gradually weakened.

3.2. Effect of Rotation Rate on Corrosion Products

Figure 4 shows the morphology of the corrosion products of P110 steel at different rotation rates. The corrosion products on the surface of specimen are flat when the rotation rate is 0 r/min, and their microstructure shows a slight honeycomb-like protrusion with holes in local areas. EDS data (

Table 4. EDS results of corrosion products on the sample surface at different rotational speeds.

Rotation Rate	Region	C	O	Ca	Fe	Cl	S	Mn
0 r/min	001	18.53	38.85	0.26	37.28	5.09	-	-
	002	28.77	44.57	0.33	24.33	2.00	-	-
60 r/min	001	27.31	41.98	13.99	15.88	-	0.42	0.40
	002	23.89	58.08	13.93	4.09	-	-	-
120 r/min	001	25.49	46.66	13.90	13.95	-	-	-
	002	25.02	57.15	13.33	4.50	-	-	-
180 r/min	001	22.91	16.81	3.47	55.73	-	-	1.09
	002	28.99	49.92	4.45	16.64	-	-	-

) and XRD spectra (Figure 5) show that the corrosion products on the surface of specimen are predominantly Fe-C compounds and FeCO₃, with very low contents of Ca compounds. Under flowing environments, as shown in Figure 4b–d, the corrosion products on the surface of the specimens show lamellar products deposited on the porous structure. As the rotation rate increases, the crystal form of lamellar products develops more regularly. EDS shows that the Ca content in the corrosion scales is approximately 13% at 60 and 120 r/min, and 4% at 180 r/min. XRD shows that the corrosion products on the surface of the specimen are predominantly composed of Fe_xCa_1−xCO₃ at 60 and 120 r/min, with no CaCO₃ particles being detected. This finding indicates that the binding between CaCO₃ and the matrix or corrosion products is weak.

3.3. Effect of Rotation Rate on Corrosion Morphology

Figure 6 shows the corrosion morphology of the specimens at different rotation rates. The surface of the specimens is uniformly corroded in the static environment (0 r/min). Pits are observed in local areas, and the largest pit is about 15 μm deep. The corrosion morphology is similar at 60 r/min, 120 r/min, and 180 r/min, showing a honeycomb- protrusion shaped corrosion morphology. No evident corrosive pitting is found on the surface of the specimen in the fluid environment. Some non-corroded areas are still observed in the crown top at the 60 r/min speed.

4. Discussion

4.1. Effect of Ca²⁺ Ion on the Properties of the Solution and Interface Reaction Rate

Table 5. pH test results of the test solution.

Concentration of Ca2+ in the Solution (g/L)	Before Adding CO2	After Adding CO2 for 120 min
0	4.93	3.72
7.5	5.72	3.60

shows the pH value of the NaCl solution. For a single NaCl solution without CaCl₂, its pH value should theoretically be 7. However, the measured value is far lower than 7. The NaCl used in the experiments is AR reagent, but impurities in the reagent lower the pH of the solution remarkably. The solution containing 7.5 g/L Ca²⁺ has a higher pH value than that of the solution without Ca²⁺ ions. This is related to the decreased NaCl content in the solution and the corresponding reduction in impurities. After CO₂ is injected, the pH values of the two test solutions decrease remarkably, and the pH value of the solution containing Ca²⁺ decreases more significantly than that of the solution without Ca²⁺. This result indicates that the presence of Ca²⁺ aggravates the acidification of the CO₂ solution.

When CO₂ gas is injected into the solution, the following dissolution reactions occur:

$${\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{HCO}}_3^{-}+{\mathrm{H}}^{+}$$

(2)

$${\mathrm{HCO}}_3^{-}\to {\mathrm{CO}}_3^{2-}+{\mathrm{H}}^{+}$$

(3)

$${\mathrm{Ca}}^{2+}+{\mathrm{CO}}_3^{2-}\to {\mathrm{CaCO}}_3$$

(4)

The low solubility of CaCO₃ promotes the continuous progress of Reaction (4), thus leaving increased H⁺ in the solution and a decreased solution pH.

Figure 7 shows the polarization curves of P110 steel in the 128 g/L Cl⁻ solution and in the 7.5 g/L Ca²⁺ + 128 g/L Cl⁻ saturated CO₂ solution. The cathodic and anodic corrosion current densities increase in a certain potential range with the addition of Ca²⁺ ions. The effect on the cathodic corrosion current is more significant than that on the anodic corrosion current. This finding may be because the addition of Ca²⁺ disturbs the equilibrium of the CO₂-H₂O system and promotes the secondary ionization HCO₃⁻.

4.2. Effect of Rotation Rate on the Protectiveness of Product Scales

Figure 8 shows the polarization curves and EIS impedance curves of P110 steel immersed in this environment for 72 h at different flow rates. The experimental environment is as follows: 90 °C, saturated CO₂, and 7.5 g/L Ca²⁺ + 128 g/L Cl⁻ solution.

For the four groups of specimens immersed for 72 h, the anodic dissolution current density is less affected by the corrosion potential, indicating that a corrosion scale forms on the surface of specimen and inhibits the diffusion of ions. The corrosion is predominantly controlled by resistance polarization. According to the fitting results listed in

Table 6. Corrosion electrochemical parameters of P110 steel at different flow rates.

Flow Rate (r/min)	Ecorr (VSCE)	Icorr (μA/cm2)
0	−0.754	7.62
60	−0.812	11.21
120	−0.856	21.10
180	−0.775	17.05
0	−0.754	7.62

, 72 h of continuous immersion does not form effective protective film on the surface of the specimen. The research specimens achieved their maximum corrosion current density (21.10 μA/cm²) at 120 r/min and their minimum corrosion current density (7.62 μA/cm²) in a static environment (0 r/min).

As shown in Figure 8b, the high-frequency region of the impedance curve is controlled by electrode reaction kinetics, whereas the low-frequency region is controlled by the diffusion of the reactions or products of the electrode reaction. As the rotation rate increases, the diffusion tail in the impedance curve gradually diverges from the 45° direction, which is related to the thickening of the corrosion scales on the surface of sample. The impedance data are fitted with the equivalent circuit shown in Figure 9, and fitting results are listed in

Table 7. EIS parameters of P110 steel.

Rotation Rate (r/min)	0 r/min	60 r/min	120 r/min	180 r/min
Rs (Ω·cm2)	1.122	1.267	1.109	1.149
Qdl (F·s1−n/cm2)	0.0009969	0.001591	0.001915	0.008068
n	0.9281	1	0.9316	0.7802
Rct (Ω·cm2)	43.24	31.42	28.89	46.03
W (s0.5/Ω2·cm2)	0.008359	0.0112	0.02337	0.05519
Qf	0.03831	0.005828	0.008057	0.03483
Rf	736	0.4919	13.27	73.18

. The difference in the charge transfer resistances value (R_ct) is slight in four different environments, and the value in the 120 r/min corrosion system is relatively small. For the resistance of the corrosion product scale (R_f), the maximum value (736 Ω/cm²) and minimum resistance value (0.4919 Ω/cm²) are observed in 0 and 60 r/min corrosion systems, respectively. This result indicates that the protective effect of the corrosion scales becomes weak when fluid state change from static condition to dynamic condition.

4.3. Effect of Rotation Rate on the Corrosion Behavior of P110 Steel in High-Ca²⁺ System

According to the kinetic molecular theory, two molecules must collide for a reaction to occur. Therefore, the reaction rate is proportional to the number of collisions per unit volume in unit time; that is, the number of collisions per unit volume in unit time is proportional to the concentration product. Thus, the consumption rate of reactants is directly proportional to the concentration product.

The reaction of sediment CaCO₃ is a first-order reaction. The corresponding rate equation can be described as:

$$V_{{\mathrm{CaCO}}_3}=K{C}_{{\mathrm{Ca}}^{2+}}{C}_{{\mathrm{CO}}_3{}^{2-}}$$

(5)

where K refers to the reaction rate constant. At a certain temperature, the rate constant is fixed.

The reaction of FeCO₃ is a multi-order reaction. The most simplified second-order reaction is as follows:

$$\mathrm{Fe}\to {\mathrm{Fe}}^{2+}+2\mathrm{e}$$

(6)

$${\mathrm{Fe}}^{2+}+{\mathrm{CO}}_3^{2-}\to {\mathrm{FeCO}}_3$$

(7)

The corresponding rate equation can be described as:

$$V_{\mathrm{FeCO}3}=K_1{K}_2{C}_{{\mathrm{Fe}}^{2+}}{C}_{\mathrm{CO}{3}^{2-}}$$

(8)

where K₁ and K₂ refer to reaction rate constants of intermediate reactions (5) and (6), respectively.

In the experiment, as shown in Equations (5) and (8), the Ca²⁺ in the solution has a high concentration in the early stage, and the formation rate of V_CaCO3 is higher than that of V_FeCO3, and CaCO₃ is preferentially deposited on the surface of specimen. In the subsequent co-sedimentation of CaCO₃ and FeCO₃, Ca²⁺ may be substituted by the Fe atoms in FeCO3 to form a Fe_xCa_1−xCO₃ chemical compound because CaCO₃ and FeCO₃ have similar hexagonal lattice structures and more K_CaCO3 than K_FeCO3 is involved in the sedimentation of CaCO₃ and FeCO₃.

The crystal forms of CaCO₃ include three structures, namely calcite, aragonite, and azagonie. With unstable thermodynamics, aragonite and azagonie gradually transform into calcite, thereby leading to the destruction of CaCO₃’s sedimentation structure. The impact and peeling of the fluid aggravate the destruction of CaCO₃’s sedimentation structure.

In the experiment, the sheet specimen was placed vertically in the solution. In the static environment (0 r/min), a large amount of CaCO₃ generated early precipitates at the bottom of the autoclave, whereas an extremely small amount of CaCO₃ formed a uniform and thin adsorption layer on the surface of P110 steel due to the adsorption effect. The loosely adsorbed CaCO₃ layer cannot effectively inhibit the development of corrosion. With the local desorption, fracture, and peeling of CaCO₃ layer, the dissolution of Fe is accelerated. At the same time, the corrosion potential of carbide (FeC) in the P110 steel (Figure 1) is higher than that of the matrix Fe, thereby forming a carbide/Fe micro-battery system in the corrosion solution. Fe is used as an anode to dissolve ions, whereas carbides remain as a cathode. After Fe atoms enter the solution in the form of Fe²⁺, carbides are deposited onto the surface of the sample. The corrosion scales with complex components (containing carbides, CaCO₃, FeCO₃ and Fe_xCa_1−xCO₃) and with a loose structure cannot effectively hinder the access and exit of corrosive ions. For example, Cl⁻ with a small radius passes through the mixed scale to form the corrosive pitting with autocatalytic effect on the surface of the matrix. The corrosive pitting develops along the depth and forms pits on the surface of the matrix.

The effect of fluid medium on corrosion predominantly focuses on two aspects, i.e., accelerating the transfer of ions and the peeling or hardening of the surface of the specimen. The addition of Ca²⁺ leads to the acidification of the solution. Flow promotes the access of corrosive ions and the exit of product ions, thereby aggravating the dissolution of interface metals. The flow rate also intensifies the formation of product scale on the surface of the specimen. After 72 h of corrosion, the impedance test results show that the impedance value of the scale in the 60 r/min corrosion system is lowest, whereas that in the 180 r/min corrosion system is highest. This result indicates that the effect of fluid on the corrosion rate of P110 steel in high-Ca²⁺ and high-Cl⁻ environment is jointly determined by an increasing interface reaction rate and increasing scale resistance.

In the fluid environment, fluid entrainment will aggravate the deposition amount and deposition density of CaCO₃ on the surface of the specimen (Figure 10a), with the dense CaCO₃ adsorption layer undergoing phase change, bubbles, and damage in the late stage (Figure 10b). Given that the surface tension in the center of the bubble is the largest, the damage diffuses from the center to the periphery. The erosion of fluid accelerates the peeling of the product scale in the bubbling area. After the center of the bubbling area is damaged, the fresh surface of the matrix in the damaged area is exposed to the solution environment, and FeCO₃ is generated (Figure 10c). If the flow rate increases, the peeling degree increases, and more fresh surfaces are exposed. As such, the generation rate of FeCO₃ is also higher. Flow increases the probability of the contact between Ca²⁺ and the FeCO₃ ions in the solution. With the increase in the fluid flow rate, the generation rate of Fe_xCa_1−xCO₃ increases, and the integrity of the crystal also increases. In terms of FeCO₃, a higher generation rate indicates a high deposition speed. If increased FeCO₃ is deposited, the scale becomes dense. A high FeCO₃ generation rate in the center of the bubbling area promotes the preferential formation of dense Fe_xCa_1−xCO₃ scale. In the surrounding area of the bubble, the mixed products of CaCO₃ and FeCO₃ are weakly peeled. Porous corrosion products cannot effectively hinder the progress of corrosion but accelerates the development of corrosion. Therefore, a coronal corrosion morphology is formed on the surface of the specimen.

In solutions with high Ca²⁺ and Cl⁻ concentrations, the fluid accelerates the transfer of corrosion ions and peels and rebuilds the product layer of the specimen. Under static condition, ions are transferred by diffusion, and the loose and intact product layer has a greater impeding effect on the ions with a lower migration rate (the value of Rf is 736 Ω· m²). The average corrosion rate of P110 steel is lower. Under flow conditions, the migration rate of ions increases with the flow rate, and the product layer is peeled by the shear stress from the fluid. The peeling and rebuilding of the products layer is not linear as the flow rate. Therefore, the corrosion rate of P110 steel in solutions with high Ca²⁺ and Cl⁻ concentrations increases in the early stage and decreases with as the flow rate increases, as shown in Figure 3.

5. Conclusions

This paper analyzes the corrosion behavior of P110 steel at different flow rates by simulating an underground environment with high temperature and high pressure. The following results were obtained:

• As the fluid flow rate increases, the corrosion rate of P110 steel increases, but the increasing trend gradually slows down.
• In high-Ca²⁺ and high-Cl⁻ environments, the effect of flow rate on the corrosion behavior of P110 steel in high-Ca²⁺ and high-Cl⁻ environment is realized through the flow rate accelerating ion transfer and constructing a fixed scale. The presence of Ca²⁺ promotes the acidification of the solution and accelerates the dissolution of the P110 steel. The formation of Ca_xFe_1−xCO₃ decreases the barrier of the corrosion film. The electric resistance of the barrier film reduces as the Ca content increases in the film.
• The flow state decreases the probability of corrosive pitting on the P110 steel. In static high-Ca²⁺ and high-Cl⁻ environments, corrosive pitting is observed on the surface of P110 steel. In the dynamic environment, the corroded surface of P110 steel is coronal, and no evident corrosive pitting is observed.
• From an engineering perspective, the injection–production operation can implement a suitable higher flow rate in high-Ca²⁺ and high-Cl⁻ environments. In addition, removing corrosion scales on the downhole tubular goods in time are necessary to prevent pitting.